With the development of computer technologies in recent years, a technique of constructing a 3-dimensional image from multi-slice images taken by CT, MRI, and the like has become known. In addition, it is now possible to form a slice image at an arbitrary point in the constructed 3-dimensional image (see, for example, Patent Documents 1 through 4 specified below).    Patent Document 1: Japanese Unexamined Patent Publication No. 2005-253518    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-188149    Patent Document 3: Japanese Unexamined Patent Publication No. 2004-049753    Patent Document 4: Japanese Unexamined Patent Publication No. 08-106546